The balloon, which was launched on December 28, 2008, from McMurdo Station in Antarctica, is 7 million cubic feet and is said to be the largest single-cell, super-pressure, fully sealed balloon ever flown. When the project--which NASA is conducting in coordination with the National Science Foundation--is completed, the space agency should have a 22 million cubic foot balloon to work with.

NASA said that long-duration high-altitude balloon missions are far more cost-effective than satellites and that a chief benefit is that the instruments used can be easily retrieved and re-used.

The test flight made it to an altitude of 111,000 feet and has been at or near that height for 11 days so far.

"The flight tested the durability and functionality of the scientific balloon's unique pumpkin-shaped design and novel material," NASA said in a statement. "The material is a special lightweight polyethylene film, about the thickness of ordinary plastic food wrap."